The parison molding cavity at the injection molding station of a typical injection blow molding machine is formed in part by a pair of superimposed mold halves that split or separate along the center line of each gate opening and parison cavity. Hot melt injection nozzles have discharge tips that are seated in the gate openings. To remove a set of newly formed parisons from the mold, the upper mold halves are raised off the bottom mold halves, and the cores that carry the new parisons are then lifted and rotated out of the mold. A new set of cores is placed in the cavities of the bottom mold halves and the mold is closed, creating a diametrical sealing relationship between each nozzle tip and the wall of its gate opening and preparing the mold cavities to receive hot melt through the nozzles.
For a variety of reasons, it is difficult to maintain both the upper and lower mold halves at uniform temperatures at all times. Consequently, the upper and lower mold halves experience different degrees of expansion and contraction, and perfect alignment between upper and lower mold halves is rarely achieved. This condition is aggravated by the fact that the hot injection nozzles typically run at significantly higher temperatures than either of the mold halves.
These thermal expansion and misalignment issues inherent in split parison cavity design typically produce wear and plastic leakage at the interface between the nozzle tip and the parison cavity. The typical fix involves replacing the nozzles and repairing the gate opening region where the interface occurs.
Another problem with conventional tooling involves the start up procedure following prolonged shut down of the machine. Initially, the nozzles and the hot manifold block to which they are attached are disposed in a retracted position with the nozzles pulled back out of the mold. To begin the start up procedure, mounting bolts on a retainer block that attach each nozzle to the manifold block are intentionally loosened so that there is some slight freedom of movement of the nozzles relative to the manifold block. The manifold block and nozzles are then heated up to their operating temperature, while the mold cavities are maintained at their operating temperatures. Then, the manifold block and nozzles are pushed forward to properly seat the nozzle tips in their respective gate openings leading to the cavities. As the top mold halves are subsequently lowered against the bottom halves, the loose nozzles are engaged and realigned as need be by the lowering mold halves. Once the mold is fully closed, all of the bolts on every nozzle are retightened to secure the nozzles in their adjusted positions. Depending upon the number of mold cavities involved, this can be a laborious and time-consuming process. At shut down, the manifold and nozzles must be pulled away from the parison cavities to prevent the nozzles from binding up in the mold and/or becoming damaged as the mold halves and nozzles cool down at different rates.
There is also a problem with leakage at the base end of each nozzle and the manifold block. The repeated impact from the upper mold half during closing of the mold and effects of significant temperature differentials, as well as loosening and retightening operations of the mounting blocks for the nozzles, necessarily result in leakage problems for the rear areas of the nozzle as well as the discharge tips.